This invention relates in general to sewing machines, and in particular to a sewing machine for the forming of stitch groups such as a row of buttonholes.
A similar sewing machine is known from U.S. Pat. No. 3,463,101. In that machine, the workpiece is a shirt front which is laid around a deflecting bar attached to the sewing table at an angle to the sewing direction. The workpiece is aligned, and then clamped at the front end into a feed device which displaces the shirt front from one buttonhole sewing point to the next and at the rear end into a return device, before the sewing of the individual buttonholes can be started. After completeion of the buttonhole row, the workpiece must be unclamped. The clamping and unclamping takes relatively much time and causes long down times of the sewing machine.
An extension spring is provided as drive means for the return device, which is increasingly tensioned during the intermittent shifting of the shirt front between the sewing operations. The stepped increase of the spring tension has the result that the shirt front is subjected to a different tensile stress at each sewing point. Because of the tensile stress, the working of knit material and other stretchable materials is not possible on the known machine, because the work easily buckles. Furthermore, because of the different stretch of the shirt front at the individual sewing points depending on spring tension and type of work, uniform distances between the buttonholes are not assured. But because uniform distances between the buttonholes are an essential quality feature of clothing, special care must be given to this aspect in making production more efficient.